Merkel cells of the terminal hair follicle of the adult human scalp

A helping hand: roles for accessory cells in the sense of touch across species

During touch, mechanical forces are converted into electrochemical signals by tactile organs made of neurons, accessory cells, and their shared extracellular spaces. Accessory cells, including Merkel cells, keratinocytes, lamellar cells, and glia, play an important role in the sensation of touch. In some cases, these cells are intrinsically mechanosensitive; however, other roles include the release of chemical messengers, the chemical modification of spaces that are shared with neurons, and the tuning of neural sensitivity by direct physical contact. Despite great progress in the last decade, the precise roles of these cells in the sense of touch remains unclear. Here we review the known and hypothesized contributions of several accessory cells to touch by incorporating research from multiple organisms including C. elegans, D. melanogaster, mammals, avian models, and plants. Several broad parallels are identified including the regulation of extracellular ions and the release of neuromodulators by accessory cells, as well as the emerging potential physical contact between accessory cells and sensory neurons via tethers. Our broader perspective incorporates the importance of accessory cells to the understanding of human touch and pain, as well as to animal touch and its molecular underpinnings, which are underrepresented among the animal welfare literature. A greater understanding of touch, which must include a role for accessory cells, is also relevant to emergent technical applications including prosthetics, virtual reality, and robotics.

Merkel Cells Are Multimodal Sensory Cells: A Review of Study Methods

Merkel cells (MCs) are rare multimodal epidermal sensory cells. Due to their interactions with slowly adapting type 1 (SA1) Aβ low-threshold mechanoreceptor (Aβ-LTMRs) afferents neurons to form Merkel complexes, they are considered to be part of the main tactile terminal organ involved in the light touch sensation. This function has been explored over time by ex vivo, in vivo, in vitro, and in silico approaches. Ex vivo studies have made it possible to characterize the topography, morphology, and cellular environment of these cells. The interactions of MCs with surrounding cells continue to be studied by ex vivo but also in vitro approaches. Indeed, in vitro models have improved the understanding of communication of MCs with other cells present in the skin at the cellular and molecular levels. As for in vivo methods, the sensory role of MC complexes can be demonstrated by observing physiological or pathological behavior after genetic modification in mouse models. In silico models are emerging and aim to elucidate the sensory coding mechanisms of these complexes. The different methods to study MC complexes presented in this review may allow the investigation of their involvement in other physiological and pathophysiological mechanisms, despite the difficulties in exploring these cells, in particular due to their rarity.

Epigenetic regulation and signalling pathways in Merkel cell development

Merkel cells are specialized epithelial cells connected to afferent nerve endings responsible for light-touch sensations, formed at specific locations in touch-sensitive regions of the mammalian skin. Although Merkel cells are descendants of the epidermal lineage, little is known about the mechanisms responsible for the development of these unique mechanosensory cells. Recent studies have highlighted that the Polycomb group (PcG) of proteins play a significant role in spatiotemporal regulation of Merkel cell formation. In addition, several of the major signalling pathways involved in skin development have been shown to regulate Merkel cell development as well. Here, we summarize the current understandings of the role of developmental regulators in Merkel cell formation, including the interplay between the epigenetic machinery and key signalling pathways, and the lineage-specific transcription factors involved in the regulation of Merkel cell development.

Dermal and Intraepidermal Merkel Cell Carcinoma With Squamous Cell Carcinoma: A Report of a Rare Case With Special Reference to the Touch Dome

ABSTRACT

In skin containing hair follicles, specialized epithelial structures known as "touch domes (TDs)" are located where the Merkel cells are clustered. We explored the histogenetic relationship between intraepidermal and dermal Merkel cell carcinomas (MCCs) and investigated which transformed progenitor cells can develop into intraepidermal MCC. We encountered an association between an extremely rare case of dermal and intraepidermal MCC with squamous cell carcinoma, which was examined using standard immunohistochemical methods with various epithelial, neuroendocrine, and TD markers including several immunohistochemical markers. Differential expression levels of CK20 and CD56 were found between intraepidermal and dermal MCCs, indicating molecularly distinct MCC populations. CK15 and CK17, expressed in TDs, were partially expressed in the intraepidermal neuroendocrine component at the tumor periphery in intraepidermal MCC with squamous cell carcinoma. These differences may suggest that the origin of dermal and intraepidermal MCCs is different under pathological conditions. We hypothesize that intraepidermal MCC is derived from tissue-specific stem cells localized within TDs.

Evidence of proliferative activity in human Merkel cells: implications in the histogenesis of Merkel cell carcinoma

The cellular origin of Merkel cell carcinoma (MCC) is controversial. We previously hypothesized that MCC originates from hair follicle stem cells or Merkel cell (MC) progenitors residing within the hair follicle bulge. Examination of three cases of combined MCC led to the unexpected discovery that numerous keratin 20 (CK20)-positive MCs within the squamous cell carcinoma (SCC) component of combined MCC appeared morphologically normal with dendritic and oval shapes. Moreover, one extremely rare case of combined SCC and MCC showed both intra-epidermal and dermal MCCs. These three cases represent the first documentation of MC hyperplasia in MCC, besides various benign follicular neoplasms associated with MC hyperplasia. Therefore, to elucidate the proliferating potential of MCs and their histogenetic relationship with MCCs, we further investigated these cases based on pathological observations. We identified numerous cells co-expressing CK20 and the proliferation marker Ki-67, identical to the morphological and immunohistochemical features of normal MCs. This finding indicated that MCs can no longer be considered as pure post-mitotic cells. Instead, they have proliferative potential under specific conditions in the diseased or wounded skin, or adjacent to various skin tumors, including MCC. Intimate co-existence of two malignant cell components composed of intradermal and intra-epidermal MCCs, with the proliferation of normal-appearing MCs in the same lesion, lends support to the hypothesis that MCs and MCC cells are derived from MC progenitors residing within the hair follicle bulge. Specifically, MCCs are derived from transformed MC progenitors with potential for dual-directional differentiation towards neuroendocrine and epithelial lineages.

Carcinoid-Like/Labyrinthine Pattern in Sebaceous Neoplasms Represents a Sebaceous Mantle Phenotype: Immunohistochemical Analysis of Aberrant Vimentin Expression and Cytokeratin 20-Positive Merkel Cell Distribution

This study investigated the nature of carcinoid-like, labyrinthine, rippled, and conventional cell arrangements in sebaceous neoplasms, focusing on vimentin expression and Merkel cell distribution in sebaceous neoplasms relative to these findings in normal sebaceous units and other sebaceous conditions. Immunohistochemistry for vimentin and cytokeratin 20 (CK20) was evaluated in carcinoid-like (n = 2), labyrinthine (n = 4), rippled (n = 3), and conventional (n = 6) sebaceomas; sebaceous mantle hyperplasia (n = 1); steatocystomas (n = 5); fibrofolliculomas (n = 4); sebaceous mantleoma (n = 1); sebaceous gland hyperplasias (n = 4); sebaceous adenomas (n = 4); and sebaceous carcinomas (n = 4) as well as normal skin tissue. The sebaceous mantle and its hamartoma (fibrofolliculoma) showed weak positivity for vimentin in the basal layer of the epithelial component and contained a few CK20-positive Merkel cells within the epithelial component, whereas mature sebaceous lobules were negative for vimentin and did not contain any Merkel cells. All sebaceomas with carcinoid-like or labyrinthine pattern highly expressed vimentin. CK20-positive Merkel cells were distributed with varying numbers in carcinoid-like pattern (2/2) and labyrinthine pattern (3/4) sebaceomas, sebaceous mantle hyperplasia (1/1), steatocystomas (3/5), fibrofolliculomas (3/4), and sebaceous mantleoma (1/1). Vimentin expression and Merkel cell distribution were observed in normal sebaceous mantles and sebaceous mantle-associated lesions, which could be evidence of a sebaceous mantle nature in the limited setting of sebaceous lesions. Furthermore, carcinoid-like/labyrinthine pattern sebaceomas also showed vimentin immunoreactivity and contained Merkel cells. Therefore, carcinoid-like/labyrinthine pattern of cell arrangement in sebaceous neoplasms may represent a morphological phenotype of sebaceous mantles.

Histogenesis of pure and combined Merkel cell carcinomas: An immunohistochemical study of 14 cases

The histogenesis of Merkel cell carcinoma (MCC) has remained unresolved. Moreover, one of the questions is whether pure MCC and combined MCC represent the same histogenesis and entity. The existence of combined MCC suggests that MCC likely arise from pluripotent stem cells. Merkel cells (MC) localize within the bulge area, which is populated by hair follicle stem cells. We used hair follicle stem cell markers to investigate whether MCC share certain characteristics of these stem cells. Fourteen MCC specimens were examined histologically and immunohistochemically. There were six pure MCC and eight combined MCC. In six combined MCC, both MCC components and squamous components at least focally shared the expression of one or more of cytokeratin (CK)15, CK19 and CD200, which are hair follicle stem cell markers. On the other hand, four cases of pure MCC showed partially distinct CK19 expression, but did not show CK15 and/or CD200 expression. There was a distinct difference between pure MCC and combined MCC on the expression of hair follicle stem cell markers. The normal skin expressed CK15, CK19 and CD200 in the bulge area, whereas CK15 and CD200 were absent in the MC-rich glabrous skin and touch domes. The results led us to hypothesize that combined MCC originate from the hair follicle stem cells. We postulate that combined MCC undergo multidirectional differentiation into squamous, glandular, mesenchymal and Merkel cells. Further investigation is warranted to confirm the histogenesis of pure MCC and combined MCC.

Merkel Cells in the Vellus Hair Follicles of Human Facial Skin: A Study Using Confocal Laser Microscopy

Many cases of Merkel cell carcinoma have recently been reported, and most of them have been localized on the facial skin. In this study, we investigated Merkel cells in the vellus hair follicles of facial region to characterize these cells in human subjects. Skin specimens doubly stained with cytokeratin (CK) 20 and either protein gene product (PGP) 9.5 or vasoreactive intestinal polypeptide (VIP) were examined by confocal laser microscopy. Many of the Merkel cells in the vellus hair follicles of the facial skin were localized in the bulge area. Some of these cells were attached to nerve terminals, although most of them were not associated with them. Our results suggest that there are two types of Merkel cells in the bulge area of the vellus hair follicles of facial skin: cells wholly unassociated with the nerve terminals and cells associated with thin nerve fibers. We postulate that the former cells may be undifferentiated (immature) and the latter differentiated (mature). If this is so, there is a chance that Merkel cell carcinoma originates from the undifferentiated Merkel cells in the bulge of the vellus hair with the formation of tumor masses in the dermis and no involvement of the epidermis. The Merkel cells connected with nerve fibers may secrete endocrine substances via a regulation of autonomic nerves.

Immunohistochemical analyses point to epidermal origin of human Merkel cells

Merkel cells, the neurosecretory cells of skin, are essential for light-touch responses and may probably fulfill additional functions. Whether these cells derive from an epidermal or a neural lineage has been a matter of dispute for a long time. In mice, recent studies have clearly demonstrated an epidermal origin of Merkel cells. Given the differences in Merkel cell distribution between human and murine skin, it is, however, unclear whether the same holds true for human Merkel cells. We therefore attempted to gain insight into the human Merkel cell lineage by co-immunodetection of the Merkel cell marker protein cytokeratin 20 (CK20) with various proteins known to be expressed either in epidermal or in neural stem cells of the skin. Neither Sox10 nor Pax3, both established markers of the neural crest lineage, exhibited any cell co-labeling with CK20. By contrast, β1 integrin, known to be enriched in epidermal stem cells, was found in nearly 70 % of interfollicular epidermal and 25 % of follicular Merkel cells. Moreover, LRIG1, also enriched in epidermal stem cells, displayed significant co-immunolabeling with CK20 as well (approximately 20 % in the interfollicular epidermis and 7 % in the hair follicle, respectively). Further epidermal markers were detected in sporadic Merkel cells. Cells co-expressing CK20 with epidermal markers may represent a transitory state between stem cells and differentiated cells. β1 integrin is probably also synthesized by a large subset of mature Merkel cells. Summarizing, our data suggest that human Merkel cells may originate from epidermal rather than neural progenitors.

Merkel cell distribution in the human eyelid

Although Merkel cell carcinoma of the eyelid is reported frequently in the literature, only limited information exists about the distribution of Merkel cells in this tissue. Therefore, serial sections of 18 human cadaver eyelids (donors ages ranging between 63 and 97 years) were stained for cytokeratin 20 in various planes. The overall appearance of Merkel cells in these samples was low and mainly located in the outer root layer of the cilia hair follicles. Merkel cells were more frequent in the middle, and almost not detectable at the nasal and temporal edges. The localization is in accordance with that of Merkel cell carcinoma, but concerning the scarce appearance within this adulthood group, a specific physiological role of these cells in the eyelid is difficult to establish.

Merkel cells as putative regulatory cells in skin disorders: an in vitro study

Merkel cells (MCs) are involved in mechanoreception, but several lines of evidence suggest that they may also participate in skin disorders through the release of neuropeptides and hormones. In addition, MC hyperplasias have been reported in inflammatory skin diseases. However, neither proliferation nor reactions to the epidermal environment have been demonstrated. We established a culture model enriched in swine MCs to analyze their proliferative capability and to discover MC survival factors and modulators of MC neuroendocrine properties. In culture, MCs reacted to bFGF by extending outgrowths. Conversely, neurotrophins failed to induce cell spreading, suggesting that they do not act as a growth factor for MCs. For the first time, we provide evidence of proliferation in culture through Ki-67 immunoreactivity. We also found that MCs reacted to histamine or activation of the proton gated/osmoreceptor TRPV4 by releasing vasoactive intestinal peptide (VIP). Since VIP is involved in many pathophysiological processes, its release suggests a putative regulatory role for MCs in skin disorders. Moreover, in contrast to mechanotransduction, neuropeptide exocytosis was Ca²⁺-independent, as inhibition of Ca²⁺ channels or culture in the absence of Ca²⁺ failed to decrease the amount of VIP released. We conclude that neuropeptide release and neurotransmitter exocytosis may be two distinct pathways that are differentially regulated.

The presence of Merkel cells and CD10- and CD34-positive stromal cells compared in benign and malignant oral tumors

BACKGROUND

To describe sequential changes in Merkel cells (MC), and CD10(+) and CD34(+) stromal cells (SC) during the transition from benign to malignant oral lesions and correlate with clinicopathologic parameters.

MATERIALS AND METHODS

Changes in cytokeratin 20-positive (CK20(+)) Merkel cells, CD10(+) and CD34(+) SC were immunohistochemically examined in specimens of 28 oral verrucous carcinomas (VC), 32 squamous cell carcinomas (SCC) and 36 benign squamous lesions (BSL). Immunoreactivity and localized inflammation were measured quantitatively and/or semiquantitatively, and between-group results were statistically compared.

RESULTS

The mean number of CD34(+) SC was significantly lower in VC (57.36) and SCC (33.81) than BSL (351.56, P < 0.001). However, the three tumor types had similar staining level and number of CD10(+) SC. We found a significant difference in the density of MC between BSL and VC (P < 0.001) or SCC (P < 0.001). The number of CK20(+) MC was significantly lower in highly inflamed specimens than mildly inflamed specimens (P = 0.001).

CONCLUSION

CD34(+) SC and to a lesser extent MC, but not CD10(+) SC, reveal statistically different density during the transition from benign to malignant oral lesions. The correlations between the CD34(+) SC expression and squamous lesions may be associated with epithelial dysplasia and/or tumor invasion.

Immunophenotyping of the human bulge region: the quest to define useful in situ markers for human epithelial hair follicle stem cells and their niche

Since the discovery of epithelial hair follicle stem cells (eHFSCs) in the bulge of human hair follicles (HFs) an important quest has started: to define useful markers. In the current study, we contribute to this by critically evaluating corresponding published immunoreactivity (IR) patterns, and by attempting to identify markers for the in situ identification of human eHFSCs and their niche. For this, human scalp skin cryosections of at least five different individuals were examined, employing standard immunohistology as well as increased sensitivity methods. Defined reference areas were compared by quantitative immunohistochemistry for the relative intensity of their specific IR. According to our experience, the most useful positive markers for human bulge cells turned out to be cytokeratin 15, cytokeratin 19 and CD200, but were not exclusive, while beta1 integrin and Lhx2 IR were not upregulated by human bulge keratinocytes. Absent IR for CD34, connexin43 and nestin on human bulge cells may be exploited as negative markers. alpha6 integrin, fibronectin, nidogen, fibrillin-1 and latent transforming growth factor (TGF)-beta-binding protein-1 were expressed throughout the connective tissue sheath of human HFs. On the other hand, tenascin-C was upregulated in the bulge and may thus constitute a component of the bulge stem cell niche of human HFs. These immunophenotyping results shed further light on the in situ expression patterns of claimed follicular 'stem cell markers' and suggest that not a single marker alone but only the use of a limited corresponding panel of positive and negative markers may offer a reasonable and pragmatic compromise for identifying human bulge stem cells in situ.

Multipotent skin-derived precursors: adult neural crest-related precursors with therapeutic potential

We previously made the surprising finding that cultures of multipotent precursors can be grown from the dermis of neonatal and adult mammalian skin. These skin-derived precursors (SKPs) display multi-lineage differentiation potential, producing both neural and mesodermal progeny in vitro, and are an apparently novel precursor cell type that is distinct from other known precursors within the skin. In this review, we begin by placing these findings within the context of the rapidly evolving stem cell field. We then describe our recent efforts focused on understanding the developmental biology of SKPs, discussing the idea that SKPs are neural crest-related precursors that (i) migrate into the skin during embryogenesis, (ii) persist within a specific dermal niche, and (iii) play a key role in the normal physiology, and potentially pathology, of the skin. We conclude by highlighting some of the therapeutic implications and unresolved questions raised by these studies.

Merkel cells

Merkel cells are post-mitotic cells scattered throughout the epidermis of vertebrates. They are particularly interesting because of the close connections that they develop with sensory nerve endings and the number of peptides they can secrete. These features suggest that they may make an important contribution to skin homeostasis and cutaneous nerve development. However, these cells remain mysterious because they are difficult to study. They have not been successfully cultured and cannot be isolated, severely hampering molecular biology and functional analysis. Merkel cells probably originate in the neural crest of avians and mammalians, and their "spontaneous" appearance in the epidermis may be caused by a neuron-independent epidermal differentiation process. Their functions are still unclear: they take part in mechanoreception or at least interact with neurons, but little is known about their interactions with other epidermal cells. This review provides a new look at these least-known cells of the skin. The numerous peptides they synthesize and release may allow them to communicate with many cells other than neurons, and it is plausible that Merkel cells play a key role in skin physiology and physiopathology.

Hair follicle stem cells: walking the maze

The discovery of epithelial stem cells (eSCs) in the bulge region of the outer root sheath of hair follicles in mice and man has encouraged research into utilizing the hair follicle as a therapeutic source of stem cells (SCs) for regenerative medicine, and has called attention to the hair follicle as a highly instructive model system for SC biology. Under physiological circumstances, bulge eSCs serve as cell pool for the cyclic regeneration of the anagen hair bulb, while they can also regenerate the sebaceous gland and the epidermis after injury. More recently, melanocyte SCs, nestin+, mesenchymal and additional, as yet ill-defined "stem cell" populations, have also been identified in or immediately adjacent to the hair follicle epithelium, including in the specialized hair follicle mesenchyme (connective tissue sheath), which is crucial to wound healing. Thus the hair follicle and its adjacent tissue environment contain unipotent, multipotent, and possibly even pluripotent SC populations of different developmental origin. It provides an ideal model system for the study of central issues in SC biology such as plasticity and SC niches, and for the identification of reliable, specific SC markers, which distinguish them from their immediate progeny (e.g. transient amplifying cells). The current review attempts to provide some guidance in this growing maze of hair follicle-associated SCs and their progeny, critically reviews potential or claimed hair follicle SC markers, highlights related differences between murine and human hair follicles, and defines major unanswered questions in this rapidly advancing field.

Changes in the number of Merkel cells with the hair cycle in hair discs on rat back skin

BACKGROUND

Hair discs are known to contain a large number of Merkel cells and are ideal for investigating Merkel cell biology. Hair follicles, which are important elements of hair discs, undergo unique cyclical morphological and biological changes.

OBJECTIVES

To define the relationships between the number and the morphology of Merkel cells within the hair disc in association with the hair cycle on rat back skin.

METHODS

Merkel cells in hair discs were observed three-dimensionally using immunohistochemistry. Epidermal sheets were incubated with monoclonal murine antibody to CK20. As a result, Merkel cells in hair discs were clearly demonstrated as whole shapes and were counted under a light microscope.

RESULTS

Merkel cells in hair discs increased during the early to middle phase of anagen and decreased during the middle phase of anagen to catagen and telogen in perinatal and postnatal rat back skin. We observed the morphological variation of Merkel cells in hair discs of rat back skin, and consequently divided them into two subtypes at the light microscopic level: the oval type and the dendritic type. The number of oval-type Merkel cells was not markedly affected by the hair cycle. In contrast, the number of dendritic-type Merkel cells markedly changed with the hair cycle.

CONCLUSIONS

This difference of the hair cycle dependency between oval and dendritic-type Merkel cells suggests some functional differences, such as a secretory function, related to the hair cycle.

The existence of a linear system consisting of sympathetic endings in rat skin

In a previous pilot study we suggested the novel notion that the catecholaminergic sympathetic nerve endings are non-homogeneously distributed in the rat skin. In the present study we have utilized several independent methods to determine the in vivo distribution of catecholamine-containing fibers in rat skin. Using whole body macro-autoradiography with an iodine125-labeled tyrosine, we localized the distribution of iodine-125-catecholamine in rat skin. The images on the film showed various pairs of symmetrical linear arrays running from the head through the back and to the hind limbs of the animal that we arbitrarily termed sympathetic substance lines (SSLs). The distribution of catecholamine in rat skin was also visualized by light microscopy autoradiography with tritiated tyrosine. The majority of silver grains in the sections were located among hair follicles along a band or zone. Furthermore, a modified sucrose-phosphate-glyoxilic acid (SPG) method was adapted to observe sympathetic fibers in the skin sections. Dense clusters of fluorescent nerve fibers in correspondence of arrector pili muscles (AP muscles) were located along lengthwise lines of the body, in a pattern coinciding with the linear arrays identified by macro-autoradiography. We concluded that concentrated clusters of noradrenergic nerve fibers innervate AP muscles and form a longitudinal linear system in the whole skin. These results are discussed in terms of physiological functions associated with hair follicles, sensory signal pathways and Meridians in Chinese traditional medicine.

Human Merkel cells – aspects of cell biology, distribution and functions

Human Merkel cells were first described by Friedrich S. Merkel in 1875 and named "Tastzellen" (touch cells) assuming a sensory touch function within the skin. Only ultrastructural research revealed their characteristics such as dense-core granules, plasma membrane spines and dendrites as well as a loosely arranged cytoskeleton. Biochemical analysis identified the expression of very specific cytokeratins (most notably CK 20) allowing the immunohistochemical detection of Merkel cells. In humans, they occur within the basal epidermis, being concentrated in eccrine glandular ridges of glabrous skin and in Haarscheiben of hairy skin, within belt-like clusters of hair follicles, and in certain mucosal tissues. Within the human skin, the dense-core granules contain heterogeneously distributed neuropeptides, some of which might work as neurotransmitters through which Merkel cells and their associated nerves exert their classical function as slowly adapting mechanoreceptors type I. This is the case in the Haarscheiben, small sensory organs containing keratinocytes with a special program of differentiation that includes the expression of CK 17 and Ber-EP4. Other peptides may act as growth factors and thus might participate in growth, differentiation and homeostasis of cutaneous structures. It is not yet clear whether the Merkel cell carcinomas, aggressive skin carcinomas, indeed arise from Merkel cells. We summarize and discuss data on the distribution, function and heterogeneity of human Merkel cells in normal and diseased skin.

Epithelial stem cells: stepping out of their niche

In this issue of Cell, have shown that two subpopulations of cells exist within the hair follicle stem cell niche. Despite being partially differentiated, clonal populations of suprabasal bulge region cells can regenerate skin and hair follicles as well as a new stem cell niche. The findings suggest that early lineage commitments of epithelial cells in the hair follicle may be reversible.

Normal human Merkel cells are present in epidermal cell populations isolated and cultured from glabrous and hairy skin sites

The Merkel cell is a highly specialized cell that primarily acts as a slowly adapting mechanoreceptor. Merkel cells are scarce in normal skin but can be identified by the expression of distinct keratin filaments. Merkel cells constitute a very unique population and many questions still remain as to their origin, number, proliferative capacity, and functions in cutaneous biology. The dissociation of epidermal cells from skin is a widely used technique to extract and culture keratinocytes. We took advantage of a two-step extraction method to quantify keratin-20-expressing Merkel cells among total cutaneous cells obtained from either hairy or glabrous skin biopsies. Flow cytometry analysis revealed that keratin-20-labeled Merkel cells represent between 3.6% and 5.7% of freshly dissociated basal epidermal cells. No significant differences were seen between samples derived from glabrous palmar and hairy anatomic sites, from children and adult, respectively. We also report on the presence of Merkel cells in primary and first subcultures of epidermal cells indicating their capacity to remain viable after extraction from skin of various anatomic sites. To our knowledge, this is the first demonstration of nontumorigenic human Merkel cells in culture in vitro. The persistence of a small number of Merkel cells in culture suggests that, with the development of appropriate culture conditions, these cells could be amplified and further studied to unravel long-standing questions relative to their paracrine function or epithelial origin.

Spatial relationship between Merkel cells and Langerhans cells in human hair follicles

The distributions of Merkel cells and Langerhans cells within human hair follicles have been reported. However, there has been no description of the relationship between Merkel cells and Langerhans cells, which were discovered by 19th century German pathologists. Merkel cells and Langerhans cells share some similar characteristics such as the localization of human hair follicles, a close association with peripheral nerves and the expression of several neuropeptides. Merkel cells were stained with CK20 or CAM5.2, while Langerhans cells were stained with CD1a or S-100 protein. We thus immunohistochemically confirmed the preferential localization of Merkel cells and Langerhans cells in normal human hair follicles. Using a double staining technique, two- and three-dimensional observations demonstrated that a small proportion of Merkel cells were closely contacted with Langerhans cells below the sebaceous gland level, presumably indicating the bulge area. Merkel cells and Langerhans cells connected directly or approached each dendrite within the basal layer of the outer root sheath. For the first time, we demonstrated a close anatomical relationship between Merkel cells and Langerhans cells within the bulge area of human hair follicles where follicular stem cells may be present. These morphological observations suggest a functional interaction between follicular Merkel cells and Langerhans cells. We herein hypothesize that Merkel cells communicate with Langerhans cells by characteristic dendrites in which some neuropeptides or cytokines may be stored.

Proneural and proneuroendocrine transcription factor expression in cutaneous mechanoreceptor (Merkel) cells and Merkel cell carcinoma

Merkel cells form part of the peripheral neuroendocrine system of the skin and act as mechanoreceptors in touch response. Merkel cell carcinoma (MCC) is a rare, aggressive disease with similarities to small cell lung cancer (SCLC), which is also of neuroendocrine origin. We previously identified a novel DNA binding protein complex specific for MCC suspension cell lines, termed Merkel nuclear factor (MNF) by its binding to the POU-IV family DNA binding consensus sequence. Here we report that MNF contains the POU-IV family member Brn-3c and that Brn-3c is expressed in normal Merkel cells. Additionally, Brn-3c protein reactivity is restricted to a subset of MCC biopsies and is not seen in biopsies revealing adherent, variant cell lines lacking neuroendocrine markers. Recently, proper development of murine Merkel cells was shown to require the proneural basic helix-loop-helix transcription factor, atonal family member, MATH1. We demonstrate a correlation between Brn-3c and HATH1 reactivity in MCC biopsies and cell lines with retention of neuroendocrine phenotype. In SCLC, the related basic helix-loop-helix transcription factor HASH1 is responsible for neuroendocrine phenotype, but HASH1 transcripts were not detected in MCC cell lines. We propose that HATH1 and Brn-3c may form a transcriptional hierarchy responsible for determining neuroendocrine phenotype in Merkel cells and that lack of Brn-3c and/or HATH1 in MCC may indicate a more aggressive disease requiring closer patient follow-up.

Merkel cells and sclerosing epithelial neoplasms

Merkel cells are normal constituents of the basal layer of the epidermis and the follicular epithelium. They have been identified in benign neoplasms with follicular germinative differentiation but seem to be absent in basal cell carcinomas (BCCs). Because sclerosing epithelial neoplasms are often sampled by small biopsies, any method that enables distinction among them would be welcome. We used immunohistochemical staining for cytokeratin 20 to assess the presence of Merkel cells in 14 cases of desmoplastic trichoepithelioma (DTE), 12 specimens of syringoma, 11 samples of morpheiform BCC, and 8 specimens of microcystic adnexal carcinoma (MAC). Merkel cells were found in association with all 14 specimens of DTE and in 1 of 11 cases of morpheiform BCC (p < 0.005) but in none of the specimens of syringoma or MAC. Our study supports previous findings that Merkel cells are seen in association with cutaneous neoplasms that are benign and of a follicular germinative origin. Although MAC may differentiate along follicular-sebaceous-apocrine lines, the absence of Merkel cells within it is consistent with its malignancy. The identification of Merkel cells in a sclerosing epithelial neoplasm of the skin points to DTE as the most likely diagnosis.

Immunohistochemical investigation of the different developmental stages of trichofolliculoma with special reference to the Merkel cell

The morphologic features of trichofolliculoma are variable, reminiscent of the anagen, catagen, and telogen phases of a normal hair follicle in its cycle. We recently described an early, fully developed stage and late stages of trichofolliculoma. Using immunohistochemical examination, we sought to demonstrate hyperplasia of Merkel cells in all three stages of trichofolliculoma. We found this to be the most striking in small lesions of the late stage. The distribution of the Merkel cells in several stages of trichofolliculoma coincided with the known arrangement of normal follicular Merkel cells during the follicular cycle. However, antibodies against neurofilaments failed to detect innervated Merkel cells, in contrast to normal follicular Merkel cells. Antibodies against Ki67 did not reveal proliferative Merkel cells in any of the trichofolliculomas, but for unknown reasons, a distinct cytoplasmic staining of Merkel cell processes sometimes occurred. Nuclear Ki67 was strongly expressed in the nuclei of follicular keratinocytes of the fully developed trichofollicullomas, whereas those at a late stage showed a markedly decreased staining pattern. Our finding of Merkel cells in all trichofolliculomas underlines their classification as hamartomas with follicular differentiation. Hyperplasia of Merkel cells, even in trichofolliculomas at a late stage, as regressing lesions might implicate hitherto unknown regulatory functions of this neuroendocrine cell.

Diffuse alopecia with stem cell folliculitis: chronic diffuse alopecia areata or a distinct entity?

A 34-year-old woman presented with an 8-year history of slowly progressive diffuse nonscarring alopecia with loss of hair density. Scalp biopsy specimens showed increased miniaturized follicles and an asymmetric wedge-shaped lymphocytic infiltrate concentrated on the stem cell-rich region at the point of entry of sebaceous ducts and at bulge-like regions of multiple follicles. Several hair bulbs emerging at the stem cell compartment also were inflamed, but the hair bulbs in the deeper dermis and subcutis were spared. I speculate whether these findings may represent a stem cell folliculitis similar to the reaction pattern previously observed in graft versus host disease and in androgenetic alopecia. The additional presence of peribulbar lymphocytic inflammation could indicate that the patient had a variant of alopecia areata. The clinical presentation of a slowly progressive diffuse alopecia without progression to clinically recognizable alopecia areata and the prominent lymphocytic inflammation involving the stem cell compartment may prompt a reexamination of similar cases currently classified as chronic diffuse alopecia areata. The concept that lymphocytes can inhibit stem cell function without destroying the stem cells themselves needs consideration.

Pagetoid Merkel cell carcinoma: epidermal origin of the tumor

We report a case of intraepidermal Merkel cell carcinoma which occurred on the face of a 76-year-old white male. This slow-growing tumor was mostly confined in the epidermis and pilosebaceous apparatus where tumor cells spread in a pagetoid fashion forming tumor cell nests. Histologically it resembled a superficial spreading melanoma. A heavy lymphocytic infiltration was seen beneath the epidermal lesion as is often seen in pagetoid melanomas. Histochemical and ultrastructural features such as the presence of cytokeratin 20, synaptophysin, neuron specific enolase, desmosomes, and dense cored granules confirmed the diagnosis of Merkel cell carcinoma. Occasional mitotic cells and many apoptotic cells were found in the tumor. Dylon positive, amyloid depositions were seen in the lower epidermis and papillary dermis; they were probably derived from apoptotic tumor cells. It was thought that apoptosis limited the speed of growth of this tumor. We believe that this is probably the most convincing case of intraepidermal Merkel cell carcinoma originating from epidermal Merkel cells or its precursors (stem cells).

Apoptotic pocket-like structures of the bulge of the terminal hair follicles of the human scalp

Terminal hair follicles of the human scalp of all ages showed apoptotic pocket-like structures in the outer root sheath of the bulge area at anagen, but not telogen phase. The occurrence of these hole structures was roughly estimated in 15% of anagen terminal hair follicles of the human scalp. The size of these apoptotic pockets was variable, ranging from pin hole-like spaces to larger structures filled with homogeneous black materials. These unusual variations were often co-localized with apoptotic degenerations and exclusively present in the presumptive bulge of anagen terminal hair follicles where arrector pili muscles were seen in the vicinity. In fact, these vacuolated structures tended to be present on the side where the major part of the arrector pili muscles anchored.

Histochemistry of the human hair follicle

Immunohistochemistry of the human hair follicle is of increasing interest in hair research. The data on antigen distribution in the different epithelial and mesenchymal structures of this unique skin appendage are superfluous now. In the present chapter, I will concentrate on selected aspects related to hair follicle differentiation, epithelial-mesenchymal interactions, proliferation and metabolic activity. Hair diseases are common. Not unusually, hair growth and structure reflect systemic disturbances. Basic knowledge of hair anatomy and histochemistry is required for their rational evaluation. Immunohistochemistry is a valuable tool for microanatomy of the hair apparatus. It offers a link between the biochemical data and structural components of hair follicles, which may help to better understand physiology of hair growth and hair diseases.

Relationship between follicular nerve supply and alopecia

The emergence of new technologies such as the combination of immunohistochemical techniques with laser scanning confocal microscopy allows one to observe and project the three-dimensional perifollicular innervation in tissue sections measuring up to 200 microns. This technology opens the door to making new discoveries about the innervation of the hair follicle. As new information is generated about the cutaneous sensory nervous system, neuropeptide expression, and the modulation of inflammatory and proliferative processes by the nervous system in the skin, it is likely this knowledge will be applied to enhance our understanding of the biology of the hair follicle in both the normal and diseased state.

Protein gene product 9.5-immunoreactive nerves and cells in human oral mucosa

BACKGROUND

Current conflicting information on the innervation of the human oral cavity indicates technical problems such as different detectability of the neural structures according to the various staining methods used and difficulties in reproducibility. The possibility of intraoral regional differences has not been properly considered.

METHODS

Human biopsies of mucosa from different intraoral regions were prepared for immunohistochemistry using protein gene product 9.5 (PGP 9.5; a marker for neuronal structures).

RESULTS

Nerves were found consistently in all the biopsies. The neural pattern showed clear regional differences. Intraepithelial nerve fibers were found in the gingiva, labia, palate, within certain fungiform papillae, and in some salivary excretory ducts. Organized nerve endings were found in varying frequencies in all but one (sublingual) region, appearing as lamellar (Meissner-like), coiled or glomerular neural structures. Merkel cell-neurite complexes were observed in the buccal, gingival, and palatal epithelia. Immunoreactive cells with many similarities to Merkel cells but without a neural connection were also encountered.

CONCLUSIONS

Conflicting results from earlier innervation studies of the oral cavity could be attributed to regional innervation differences. The distribution of the nerves also casts doubt on some of the present theories concerning the function(s) of intraoral nerves, such as the free nerve endings and the Merkel cell-neurite complexes.

KIT expression reveals a population of precursor melanocytes in human skin

Human skin is believed to harbor a reservoir population of precursor melanocytes. It has been difficult to identify these putative cells experimentally, because they lack phenotypic features that define mature melanocytes. We have evaluated expression of the KIT tyrosine kinase receptor, which is critical for melanocyte development, as a possible marker of these cells. Sections of human skin were evaluated with single- and double-immunolabeling techniques. KIT-reactive dendritic cells were identified in the basal layer of the epithelia and were most numerous in the follicular infundibula and the rete ridges. These cells were located on the epithelial side of the basement membrane and lacked expression of cytokeratin and mast cell tryptase. The location of the KIT-reactive cells was distinctly different from that of Langerhans cells (identified with anti-CD1a) or Merkel cells (identified with CAM 5.2). Within the epidermis and upper follicular infundibulum the majority of the KIT-reactive dendritic cells also coexpressed TRP-1, a marker present in differentiated melanocytes. In the deeper follicular regions, the coexpression of TRP-1 in the KIT-reactive cells was absent. Throughout the epidermis and follicle, however, the KIT-reactive cells coexpressed BCL-2, a marker known to be increased in melanocytes. Thus, KIT expression reveals a population of intraepithelial cells that have immunophenotypic characteristics of mature melanocytes within the upper epithelial regions, but lack the differentiated melanocytic phenotype within the deeper follicular regions. We propose that these KIT(+), BCL-2(+), and TRP-1(-) cells constitute a precursor melanocyte reservoir of human skin.

Two- and three-dimensional demonstrations of morphological alterations of early anagen hair follicle with special reference to the bulge area

During late telogen to early anagen secondary hair germ is newly formed by the downgrowth of a clubbed column which is indistinguishable from the bulge. Serial vertical sections demonstrated that the early anagen terminal hair follicle formed the new secondary hair germ associated with a lateral protuberance of basaloid cells which could be considered as the bulge of the new hair follicle. Interestingly, the arrector pili muscle bundle was divided into two branches, one inserted into the original clubbed end and the other into this protuberance of the secondary hair germ. CAM5.2-reactive Merkel cells were present not only in the clubbed ends of the old follicle but also in the protuberance of the new hair germ. The formation of the lateral protuberance of the new hair germ preceded the appearance of CAM5.2-reactive Merkel cells in this location. Ks19.1 immunoreactivity was observed from the clubbed end to the upper half of the new hair germ. These phenomena occurred in early anagen before the club hairs were shed. It is postulated that the early anagen hair follicle formed the area within the new hair germ equivalent to the bulge and Merkel cells either moved from the bulge of the old hair follicle or differentiated de novo from immature epithelial cells. Merkel cells or their products in the bulge may serve as attractants for the readjusting arrector pili muscle to anchor to the bulge of the new hair follicle.

Merkel cells are integral constituents of desmoplastic trichoepithelioma: an immunohistochemical and electron microscopic study

The incidence of Merkel cells has previously been investigated in a number of inflammatory and tumorous lesions of the skin. Special attention was given to tumors with follicular differentiation. In the present study we examined the localization of Merkel cells in another adnexal tumor, the desmoplastic trichoepithelioma (n = 15), as well as in its main differential diagnosis, the morpheiform basal-cell carcinoma (n = 30). Using immunohistochemical methods, we found Merkel cells as a stable constituent in desmoplastic trichoepitheliomas, but failed to detect them in morpheiform basal-cell carcinomas. These findings might therefore be an important tool in the sometimes very difficult but clinically imperative distinction between these two conditions. Furthermore, our study may be of interest in the discussion about the origin of desmoplastic trichoepitheliomas. High numbers of Merkel cells in desmoplastic trichoepitheliomas indicate a bulge-derived origin of this adnexal tumor, since high numbers of Merkel cells, especially in the bulge, were recently discovered. Although the significance of Merkel cell hyperplasia in desmoplastic trichoepithelioma is not presently understood, a regulatory role of the Merkel cell in growth and development of this adnexal tumor is suggested.

Merkel cells do not require trophic maintenance from the nerves in adult human skin

A 34-year-old Japanese man with hereditary sensory neuropathy was examined to evaluate the distribution, density and inter-relationship between Merkel cells and peripheral nerves in the skin. An epidermal sheet of affected plantar skin showed numerous CAM 5.2-reactive Merkel cells, whereas PGP 9.5-reactive peripheral nerves were completely absent in the epidermis and dermis. These findings strongly suggest that Merkel cells do not require trophic maintenance from nerves in adult human skin.

Localization of Merkel cells at hairless and hairy human skin sites using keratin 18

Merkel cells are neurosecretory cells of the skin with epithelial features such as desmosomes and expression of keratins 8, 18, 19, and 20. Merkel cells are scarcely distributed in adult human skin. Although they are present in hair follicles, their density is higher at hairless anatomic sites such as palms and soles. These cells are often innervated by sensory nerve fibers and are thought to be specialized mechanosensory skin receptor cells. However, their precise origin and function are not clearly established. The aim of this study was to localize Merkel cells in human hairless and hairy skin by immunohistochemistry with antibodies Ks18.174 and Ks19.1 directed against keratins 18 and 19, respectively. In glabrous skin of palm and sole, Merkel cells have been localized at the bottom of the rete ridges, in the epidermal basal layer. To study Merkel cell distribution at hairy anatomic sites, we have chosen breast skin, a tissue containing small hair follicles typical of those covering most of the body's surface. Merkel cells were present in the interfollicular epidermis. In hair follicles, they have been identified in the isthmus region.

Scanning electron microscopic observations of extracted terminal hair follicles of the adult human scalp and eyebrow with special references to the bulge area

Scanning electron microscopic studies of human terminal hair follicles of the scalp and eyebrow have previously been limited to the hair shaft. In this study we investigated EDTA-treated extracted whole hair follicles in which most of the basal cell surface of the outer root sheath was well preserved. In the bulge area of scalp follicles there were many knob-like or villous projections. These were located in some specimens on one side of the follicle, while in others they were located around the entire circumference of the follicle. These projections were thought to represent the anchoring points of the branched follicular end of the arrector muscles. Ring-like elevations with groove-like depressions above and below were also observed surrounding the entire follicle. These were thought to represent the track of circumfollicular arrector muscles which depressed the outer root sheath when they repeatedly contracted. Most anagen eyebrow follicles showed morphological variations in the bulge area such as lattice-window-like structures and undulation patterns. In telogen follicles, the bulge became indistinguishable from the clubbed end. The lower end of these telogen follicles showed irregularly shaped bulge areas, but did not show lattice-window-like structures or undulation patterns as observed in anagen follicles. Interestingly, a hole was found in some bulge areas of both anagen and telogen follicles. Serial vertical sections of follicles revealed invaginated areas, which seemed to correspond to the openings seen in whole mounts. In vertical sections of eyebrow follicles some keratinocytes of the outer root sheaths of the bulge area were seen to be melanized to various extents. This phenomen was independent of hair cycle phase.

Cutaneous cysts of Gardner's syndrome are similar to follicular stem cells

Cutaneous cysts from a patient with Gardner's syndrome were histopathologically studied in detail. The cysts were, by and large, indistinguishable from ordinary epidermal cysts. However, several distinctive features were found: 1) epidermal or trichilemmal keratinization, 2) mature sebaceous glands connected with the cyst wall, 3) hair matrix-like structures associated with dermal papilla cells, 4) pilomatricoma-like changes, 5) intraluminal masses or pericystic deposits of shadow cells variably accompanied with foreign body reaction, 6) foreign body reaction or masses of shadow cells lining completely eroded cysts, 7) the presumptive bulge area, and 8) epithelial islands adjacent to the cyst. Each cutaneous cyst showed a variable combination of the findings described above. Foci of the basal layer of some cyst walls or epithelial islands were immunohistochemically stained with CK19, where CK20-reactive Merkel cells were also present. These findings were consistent with those of the bulge area. Unexpectedly, desmin-reactive muscle bundles, presumably indicating arrector pili muscle, were observed along the cyst wall. Our observations suggest that Gardner's cysts may be derived from putative follicular stem cells which reside in the bulge area.

Immunohistochemical demonstration of keratin 19 expression in isolated human hair follicles

We examined keratins 19 and 8 in extracted human hair follicles using monoclonal antibodies Ks19.1 and CAM5.2, respectively. Ks19.1 reactivity was found in the bulge and infundibulum. Ks19.1(+) cells were dense in the bulge of vellus and intermediate hair follicles. The intact bulge of terminal hair could not be extracted, but the presence of Ks19.1(+) cells was confirmed in transverse sections. Infundibular Ks19.1(+) cells exhibited a dense network pattern of staining in terminal hair follicle, but only a few cells were labeled in vellus and intermediate hair follicles. CAM5.2(+) cells, i.e., Merkel cells, were found in the same locations as Ks19.1(+) cells but were less dense. These patterns of distribution and staining density were not influenced by different phases of hair cycle. Sequential staining of Ks19.1 and CAM5.2 in the same hair follicle demonstrated that the same cells could be reactive for both. However, considering the large number of Ks19.1(+) cells and rather small number of CAM5.2 in the same locations, it was assumed that only a subset of Ks19.1(+) cells are Merkel cells. It was postulated that the bulge area of human adult hair follicles houses embryonic pluripotential cells characterized by stem cells and post-stem cells and that the Merkel cells in the bulge area arise from these immature cells and may play a role in the maintenance and stimulation of this group of immature cells.

Two- and three-dimensional observations of human terminal and vellus hair follicles

Extracted human terminal and vellus hair follicles were prepared by EDTA and examined by light and electron microscopes. These samples were morphologically well preserved and enabled us to observe 3-dimensional (3-D) views of the outer surfaces of hair follicles. Interestingly, the bulge areas of terminal and vellus hair follicles showed several morphological variations, such as knob-like swellings and villous projections, independent of hair cycle. Moreover, skirt-like structures were furnished in the small vellus hair follicles independent of hair cycle, but not terminal and large vellus hair follicles. These morphological variations of human hair follicles were confirmed to genuinely represent 3-D views by means of the conventional transverse and vertical sections of the human skin.